Base material decomposition is a generally known method for analyzing image data sets that have been generated with multi-spectral computerized tomography (CT) imaging, in particular have been recorded with a dual-energy CT scan. The method of base material decomposition has been described by Alvarez and Macovski in [AM76]. Base material decomposition can be performed, in particular, in the raw data space (projection-based) or in the image space (image-based).
DE 10 2006 009 222 A1 discloses a method and a device for determining the concentration of a substance in a body material by way of multi-energy computerized tomography.
With base material decomposition CT images can be calculated in which the base materials are selectively displayed. For example, with water/iodine decomposition, water is displayed fully in the water base material image and iodine fully in the iodine base material image. All other materials can be displayed partly in one image and partly in the other image.
The three-material decomposition known to a person skilled in the art represents an improvement in this traditional base material decomposition. The two base materials can be present in a shared surrounding material, such as, for example, soft tissue. The contrast agent image can then contain, for example, only iodine, while, for example, soft tissue and calcium or bones can be seen in the virtual non-contrast image.
An intensification of the image noise compared to the original images can occur in the result images with material decompositions of this kind. Noise reduction methods can be used to reduce this effect. Noise reduction is possible, for example, by using statistical information from the data of the image points adjacent to the image point, in particular neighboring voxels, for evaluation of an image point, in particular an image voxel. A noise reduction of this kind is a non-local operation therefore.
The image quality of the generally known base material decomposition can sometimes be affected by materials in the non-physical region of the decomposition if no additional correction is applied.
The physically expedient region of a base material decomposition may be taken to mean when the CT value of the second energy is plotted in a graph against the CT value of the first energy for a region in the dual-energy image, as is shown in FIG. 5. A wedge is obtained in the graph, and this corresponds to positive concentrations of the base material and therewith the physically expedient region of the base material decomposition. Further body materials can be located outside of the physically expedient region, however. When calculating base material images, these can be converted into positive and negative concentrations of the base materials. They can therefore be perceived to be disruptive.
This problem can occur in particular, with image-based three-material decomposition in blood, calcified atherosclerotic plaques and contrast medium. In this case the vascular calcification can be mathematically subtracted in order to see all other body materials and the contrast medium in the remaining image. An improved display of the vessel lumen can be achieved in this way. Negative calcium and positive iodine concentrations can, in particular, be associated with the image points relating to the fatty tissue and/or air, however. Therefore, fat and/or air on the one hand and contrast medium on the other cannot always be clearly distinguished. The presentation of vessels in the image can therefore be impaired after a calcium subtraction.
This problem can therefore occur, in particular, when materials are located outside of the physically permitted region of the base material in the case of image-based base material decomposition (with optional additional noise reduction).
A subsequent threshold value-based elimination of physically inexpediently decomposed materials cannot significantly improve the image quality in many cases since residual light edges occur due to partial volume effects and the finite range of image-based noise reduction filters.
DE 10 2011 083 727 A1 discloses a method for generating a noise-reduced CT image data set, a computing system and a CT system.